Why can't DQN be used for self-driving cars?
I'm not familiar with the ins and outs of self-driving cars, but I imagine that the action space is not discrete. For instance, the car may want to decide what angle it needs to turn (rather than left or right). The update in Q-Learning involves taking $\max_aQ(s, a)$; this is theoretically possible for a continuous action space, but it would itself require some expensive optimisation at each time step to find the maximum. It is more likely that if RL were to be applied to self-driving cars it would be through a method that easily allows for a continuous action space, like the methods detailed in this paper.
I found this survey of Deep RL for autonomous driving that you may want to look at.